Magnetic recording media such as audio and video magnetic recording tapes and computer cartridge tapes are constructed of a magnetic recording layer provided on a nonmagnetic substrate. The backside of the substrate is optionally provided with a backside coating in order to enhance the running properties of the media. The magnetic recording media coatings (backside coating and magnetic coating) are often coated in the form of a solvent-based dispersion which generally comprise a binder and a pigment dispersed in a solvent.
In the field of magnetic recording media, particularly audio and video recording tapes, data cartridge tapes, and the like, electromagnetic performance properties of a magnetic recording tape are important to overall product performance. An example of a deleterious property of a magnetic recording medium is dropout of data. Dropout is generally defined as variation (reduction) in the signal level of reproduced, tape-recorded data, resulting in an error in data reproduction. Jorgenson, The Complete Handbook of Magnetic Recording, 691 (3ed. 1988).
Data dropout has been attributed to various causes, including the presence of foreign particles on a magnetic recording tape, embossing of the magnetic layer, and other general imperfections of the magnetic recording layer. Practitioners have proposed various approaches for alleviating the problem of dropouts. For example, Japanese Patent Kokai No. SHO 56[1981]-139030 attempts to solve the problem of "output fluctuation" by adding a surfactant to the backcoat.
Some practitioners have observed that undesired texture in the backside coating may cause deterioration of electromagnetic properties. In U.S. Pat. No. 4,851,289 (Ogawa et al.), the inventors state that surface properties and electromagnetic properties are deteriorated by the "orange peel" of the backing layer caused by making the layer thick. Col. 1 lines 61-65. Ogawa et al. address these problems by providing recording media with a plurality of magnetic layers, the ferromagnetic particles in the uppermost layer having a magnetic crystal size smaller than those of any lower levels. Col. 2 lines 28-32
U.S. Pat. No. 5,114,778 (Yanai et al.) recites that an uneven orange peel texture is formed in coatings of high molecular weight compounds, based upon a convection cell known as "Bernard (sic) Cells." Benard Cells have been defined as "polymer coating defects [arising] from circulatory motion within the liquid film after it has been applied". Charles M. Hansen and Percy E. Pierce, Cellular Convection in Polymer Coatings--An Assessment, 12 Ind. Eng. Chem. Prod. Res. Develop. 67 (1973). Yanai et al. viewed this texture as beneficial, and claimed an invention comprising a magnetic recording layer, and an underlayer "composed mainly of a resin compound having on the surface thereof an uneven orange peel texture." Col. 11 lines 11-12. The object of Yanai et al. is to provide a magnetic recording medium having a constant friction coefficient after repeated use, stable running properties, excellent durability, and excellent electromagnetic characteristics. Col. 2 lines 18-21. Yanai et al. pursue these results by positioning a magnetic recording layer upon an underlayer of high molecular weight polymer that is intentionally characterized by an uneven orange peel texture appearing regularly throughout the surface of the medium. Col. 5 lines 50-55.
The paint industry has long recognized natural phenomena associated with solvent evaporation. Some of these phenomena, i.e. pigment flotation and the formation of hexagonal cells (i.e. Benard Cells), are discussed in I. Ferguson, Flotation in Paints--A Suggested Mechanism, 42 Journal of the Oil & Colour Chemists' Ass., August 1959 at 529. Being apparent to the naked eye, Benard Cells are objectionable to members of the paint industry. Thus, Ferguson attempted to explain and offer possible means of controlling Benard Cell formation.
Ferguson proposed that a mechanism for the formation of Benard Cells is a vortex action caused by solvent evaporation. In this regard, Ferguson postulated that the heat of vaporization of the solvent in the paint may give rise to a temperature gradient causing the evaporating surface to acquire a lower temperature than the remainder of the paint film and thereby producing vertical convection currents. Ferguson at 530-531.
In addition to temperature gradients, other factors are thought to influence the formation of convective currents and thereby create Benard Cells. Researchers have combined the effects of the temperature variation of surface tension (-d.sigma./dT), temperature gradient (.beta.), film thickness (d), viscosity (.mu.) and thermal diffusivity (K), to arrive at the dimensionless Marangoni number, B: ##EQU2## If a critical Marangoni number is exceeded, Benard Cells are predicted to form by surface tension effects. Hansen and Pierce supra at 67.